For what it's worth, the 16 inch cannons used on battleships during WWII used an "igniter charge" of black powder between the main smokeless load and the primer. If I remember correctly, that igniter charge was something like five kg of black powder, which supports my theory that smokeless powder needs more than just being set on fire to function correctly because a straight primer could easily have done that.
Maybe it was only two kilograms of black powder, I can't refind the info on the web, anyway, that's a pretty big charge of black powder, way more than you would need to simply ignite the smokeless charge.